1. Field of the Invention
The present invention relates to a control apparatus for an electroviscous fluid of which viscosity changes remarkably in accordance with intensity of an external electric field.
2. Discussion of the Prior Art
In an official bulletin of the Japanese patent publication NO. 144150/1988, there has been proposed a speed control apparatus wherein a speed of an actuator, such as a cylinder or the like, which is driven by an electroviscous fluid applied thereto as working oil, is controlled by controlling the intensity of an external electric field given to the electroviscous fluid.
Generally the electroviscous fluid used is a fluid in which disperse phase particles composed of silica, cellulose, various ion-exchange resin and the like are dispersed and suspended into such an electrically insulating disperse medium as silicone oil or the like. When the electroviscous fluid is subjected to the external electric field, there occurs the Winslow effect in which viscosity of the fluid is remarkably increased to induce a large shearing stress. With this Winslow effect it is possible to control the viscosity of the fluid using an electric signal easily and at a rapid speed of response. Additionally, it is known that an electric voltage for generating the external electric field may be either an alternating voltage or a direct-current voltage.
In the control apparatus which utilizes of the electroviscous fluid for generating an electric field by applying a direct-current voltage, however, particles tend to move with the lapse of time toward an electrode face and also to deposit on it. Thus, a distance between electrodes is substantially narrowed. As the result, it becomes difficult to correctly control the viscosity of the electroviscous fluid by the direct-current voltage.
For avoiding this, in an official bulletin of the Japanese patent publication NO. 144374/1989, there is proposed a control method for an electroviscous fluid in which a direct-current voltage having its sign changed alternately is applied pulsatingly.
However, there have occurred the following problems in the prior art. When the control method for the electroviscous fluid is used in which the direct-current voltage with its sign changed alternately is applied pulsatingly, a rush current Ta flows when the direct-current voltage changes suddenly, as shown in FIG. 4, resulting in an increase of a quantity of an electric current. This results in the control apparatus for the electroviscous fluid functioning electrically as a capacitor. For coping with the rush current, there has been a problem that the control apparatus, including a transformer, must be larger in size. This has also caused rise in cost of the control apparatus.
Such a large rush current also deteriorates remarkably the electrochemical properties of the electroviscous fluid. Accordingly, there is a problem that durability of the electroviscous fluid becomes bad. When consumption of a current is great, temperature of the electroviscous fluid is raised to change performance of the electroviscous fluid. Thus, it becomes difficult to control the viscosity of the electroviscous fluid accurately. As the result, there is a problem that the speed or the like of the electroviscous fluid may not be stably controlled.
In case a sinusoidal or triangular waveform pulse signal is used as an alternating pulse signal, it is possible to prevent generation of the rush current. However, usage of the sinusoidal or triangular waveform pulse signal changes an absolute value of an electric voltage value. Accordingly, there is a problem in case a cylinder is used for precise transfer, because chattering vibrations are induced on movement of the cylinder, as shown in FIG. 7. This figure indicates vibrations of a piston when the cylinder has been actuated by applying an alternating sinusoidal waveform voltage to the electroviscous fluid. Assuming that the axis of abscissas is taken as a time axis and the axis of ordinates is taken as a displacement X, a speed V and an acceleration A of the cylinder, chattering vibrations B are induced on the piston, as shown by the acceleration A.